Attenuation-free optical connection

ABSTRACT

An attenuation-free optical connection which includes a fiber optical signal fiber for transmission of an effective signal. Amplification elements are placed at intervals along the signal fiber for amplification of the effective signal and compensation for the signal fiber intrinsic attenuation. A drive element is common for the amplification elements which include a length of doped fiber which is fed via a feeding fiber by means of a ray distributor and a coupler. The drive element includes a pump station with one or more pump lasers for feeding of one or more signal fibres. Thanks to the passive amplification elements an attenuation-free connection with a minimal noise contribution is obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device at an optical communicationsnetwork which supplies a virtual attenuation-free optical connection byutilizing a driver laser which transmits an optical pump signal whichactivates amplifying elements which are placed along a signal line. Theamplifying elements give an essentially noiseless amplification byutilizing a doped fibre, and by that a compensation of the in the signalline intrinsic attenuation is obtained in a way that a virtuallyattenuation-free optical connection is obtained.

2. Discussion of the Background

A problem at transmission of optical signals in an opticalcommunications network is that often too much noise is obtained from thebig and besides sparsely located signal amplifiers. This results in arestricted transmission capacity at the transmission of optical signalsand also requires a complex and expensive terminal equipment both on theamplifier side and the receiver side.

A technology which is used today is to place fibre optical amplifierswith high amplification in special pump stations at certain points alongthe signal fibre. The pump stations contains the pump laser and anactive fibre in the same place. The amplifiers are placed comparativelysparsely, usually at a distance of 50-100 km. The high amplificationresults in that the amplifiers produce much noise and that they must bespecially equipped with isolators, filters etc to avoid disturbanceslike self-oscillations and saturation. It is too expensive to placethese amplifiers close, because of installation costs and supervisionand maintenance. U.S. Pat. No. 5,115,338 describes a system of thistype. Also U.S. Pat. No. 4,699,452 describes a similar system whichutilizes the Raman principle instead of doped fibre as amplifyingelement.

The big noise contribution from the amplifiers results in that theoptical transmitter laser must produce high output power, often via aspecial transmitter booster. High power levels results in that the fibrebecomes unlinear as signal carrier. Such an unlinear transmissionchannel leads to strong restrictions of the range, how high data ratesthat can be used, and the number of optical channels which can betransmitted in the signal fibre at the same time.

Another method is to distribute the amplification along the signal fibreby applying doped fibres at points along the signal fibre and in thisway reduce the noise contributions. At this the power of the pump laseris driven directly into one end of the signal fibre. The high pump powerwhich is required, results in further problems with unlineartransmission, as in the case above. An example of this method isdescribed in U.S. Pat. No. 5,039,199.

A similar method is to make the whole signal fibre as a low-doped fibre.This method is however impaired by the same problem. An example of thismethod is given in U.S. Pat. No. 5,058,974 and a combination of thesemethods is given in EP 0 408 394.

EP 0 387 075 shows that a doped fibre can be used in opticaltransmitters as an alternative to the semiconductor laser. The documentshows a way to more efficiently utilize the pump power necessary todrive the doped fibre.

U.S. Pat. No. 4,546,476 describes a fibre optical amplifier where thecoupler and the active fibre are arranged as one component.

SUMMARY OF THE INVENTION

The present invention solves the problems of the previously knowntechnology by placing passive amplifying elements at regular intervalsalong the signal fibre. The amplifying elements are fed via a feedingfibre by a common pump laser. This makes the signal fibreattenuation-free and gives minimal noise contribution.

The present invention consequently supplies an attenuation-free opticalconnection comprising a fibre optical signal fibre for transmission ofan effective signal. According to the invention amplifying elements areplaced at intervals along the signal fibre for amplification of theeffective signal and compensation for the in the signal fibre intrinsicattenuation. A drive element is in common for the amplification elementsfor the driving of these. Each amplification element includes preferablya lenght of doped fibre which is fed via a feeding fibre by means of aray distributor and a coupler. The drive element includes a pump stationwith one ore more pump lasers for supplying of one or more signalfibres.

The invention is defined in details in the enclosed patent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in details with reference toenclosed drawings where,

FIG. 1 is a block diagram over an attenuation-free optical connectionaccording to the invention.

FIG. 2 is a diagram over an amplifying element in details, and

FIG. 3 is a diagram over an alternative amplifying element in details.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is based on the principle of amplifying an optical signalin a fibre by introducing a bit of doped fibre. By introducing a pumpsignal in the fibre the doped fibre amplifies the optical, effectivesignal in the signal fibre making the attenuation in the signal fibrebeing compensated for.

In FIG. 1 is show n an embodiment of the present invention. The opticalconnection includes an optical fibre 1, here called the signal fibre,for transmission of the effective signal. Along the signal fibre 1, anumber of amplifying elements 2 are placed. The distance between theamplifying elements is a few kilometres. The amplifying elements aredriven by a pump station 3 which provides the amplifying elements withoptical power via a feeding fibre 4. The signal fibre 1 and the feedingfibre 4 are preferably parts of a common fibre cable. In the pumpstation there are one or more pump lasers 5. The optical power from thepump lasers is transmitted into the feeding fibre 4. At each amplifyingelement a fraction of the power in the feeding fibre is drawn off to theamplifying element via a ray distributor.

In FIG. 2 the amplifying element is shown. It consists of passiveoptical components: a ray distributor 6, a coupler 7, and a doped fibre8. The drawn pump power is via the coupler 7 brought to the signal fibre1 and is made to act on the specially doped, signal amplifying fibre 8,which is spliced into the signal fibre 1. The pump power excites thedoping ions in the signal amplifying fibre 8 which results in that theseamplify the optical effective signal. The length of the signalamplifying fibre 8 is typically about a meter.

All the amplifying elements 2 can be of the same kind. The part of thepump power which the ray distributor 5 drains depends on how manyseparations that shall be made from the pump station.

In the solution according to FIG. 1 the pump power is reduced on onehand due to losses in the feeding fibre and on the other due to the usedpower in the coupler 7. The ray distributor 6 consequently shall makeuse of different size of the power which is fed to the amplifyingelement depending on where in the chain it is located.

The amplifying elements according to the presented invention in FIG. 2consequently implies that the amplifying elements are different in sofar that the ray parts shall draw different large parts of the availablepower.

An alternative construction of the amplifying elements is shown in FIG.3. The effective signal and the optical power is introduced from theleft in the figure. The optical signal is transferred from the pumpstation in the cable 4 to the coupler 7'. In the coupler 7' the opticalpower is transferred to the signal fibre 1. The transferred pump powerin the fibre 1 is after that active in the specially doped signalamplifying fibre 8 which as previously is spliced in the signal fibre 1.The pump power excites the doping ions in the signal amplifying fibre 8which results in that these amplify the optical, effective signal. Thelength of the signal amplifying fibre 8 is for instance about a meter.

The signal is after that brought to a second coupler 7". In the coupler7" that part of the pump power which is not used in the feeding fibre isbrought back. The amplified effective signal is after that forwarded outinto the signal fibre to the next amplifying element or receiver. Thepump power which remains is also forwarded into the feeding fibre to thenext amplifying element. At the next amplifying element the feedingfibre, however, beside the used power in the first amplifying element 2will also lose some power at the transmission in the fibre.Amplification in the following amplifying element is however made in acorresponding way as has recently been described. In the solutionaccording to FIG. 3 all amplifying elements can be identically designed.No consideration need in this case to be taken to the fact that theoptical power is reduced through drainage in the amplifying elements andthrough losses in the feeding fibre 4.

As is indicated in FIG. 1 the pump station 3 also can provide a numberof signal fibres with pump power by ray distribution in the pump stationor by different pump lasers. It is convenient that an amplifying elementis placed in the pump station 3 itself, as is shown with the doped fibre8' in FIG. 1.

It is first of all the intention that the amplifying elements 2 shouldbe placed at the same distance between them along the signal fibre 1,and the drained pump power be adjusted in a way that the attentuation inthe signal fibre 1 is just compensated. Another variant is to have asomewhat higher amplification making the connection slowly amplifyingthe optical signal as this is travelling along the fibre cable.

The present invention consequently solves the problem at anattenuation-free optical connection and has more advantages comparedwith the previously known technology. The fibre cable becomesattenuation-free and gives minimal noise contribution. This results inthat one can reduce the transmission power with 20-30 dB (100-1000times,). One then has at least a margin of 20 dB to the threshold whereunlinearity starts to appear.

Elimination of unlinearity results in that one can use dispersionshifted fibre as signal fibre. One then evades complex installations fordispersion compensation.

No filters or isolators are needed, which makes the fibre cabletransparent in both directions.

One no longer has need for high power in the transmitter equipment. Thisresults in strongly changed conditions for construction and optimizationof optical high capacity systems.

With the present invention the optical transmission equipment can besimplified and becomes cheaper.

Solely passive optical components result in simple, robust and cheapimplementation.

The distance between pump stations can be increased compared with thecase where optical amplifiers at certain points are used.

Signal format, data rate, wavelenghts etc can be changed also afterinstallation of the optical cable.

Further embodiments of the invention are evident for any person withexpert knowlewdge. The invention is only restricted by the followingpatent claims.

I claim:
 1. An attenuation-free optical connection system, comprising:anoptical fiber configured to transmit an optical signal therethrough; afeeding fiber configured to transmit optical power; a plurality ofamplification elements disposed along said optical fiber and saidfeeding fiber; and a drive element coupled to said plurality ofamplification elements via said feeding fiber, and configured to supplypower to said plurality of amplification elements, wherein each of saidplurality of amplification elements is configured to amplify saidoptical signal using a predetermined portion of said power supplied bysaid drive element dependent on respective distance from said driveelement along said feeding fiber, and thereby to compensate for anintrinsic attenuation of the optical signal imparted by the opticalfiber.
 2. The optical attenuation-free optical connection system ofclaim 1, wherein:said plurality of amplification elements each include apredetermined length of doped fiber being fed from said feeding fiberthat is fed from a ray distributor and coupling to respective of saidfirst amplification element and said second amplification element by acoupler.
 3. The optical attenuation-free optical connection system ofclaim 2, wherein:the drive element includes a pump station having atleast one pump configured to feed at least the optical fiber.
 4. Theoptical attenuation-free optical connection system of claim 1,wherein:said plurality of amplification elements each include apredetermined length of doped fiber having connected on one end thereofa first coupler and a second coupler on the other end.
 5. The opticalattenuation-free optical connection system of claim 4, wherein:saidfeeding fiber couples power to each respective amplification element viaat least one of the first coupler and the second coupler to thepredetermined length of doped fiber so as to amplify the optical signalin the predetermined length of doped fiber using respectivepredetermined portion of said power.
 6. The optical attenuation-freeoptical connection system of claim 4, wherein:the drive element includesa pump station having at least one pump configured to feed at least theoptical fiber.
 7. The optical attenuation-free optical connection systemof claim 5, wherein:said feeding fiber is configured to couple the powerto said first coupler; and a residual portion of said power that is notused to amplify the optical signal in the predetermined length of dopedfiber is fed back to said feeding fiber by said second coupler.
 8. Theoptical attenuation-free optical connection system of claim 1,wherein:said drive element comprises a third amplification element. 9.The optical attenuation-free optical connection system of claim 1,wherein:each of said plurality of amplification elements is disposed atregular intervals along said optical fiber, and is configured to outputsubstantially equal amounts of power.
 10. The optical attenuation-freeoptical connection system of claim 1, wherein:said plurality ofamplification elements each include at least a meter of doped fiber; andeach amplification element is separated from a neighboring amplificationelement by at least a kilometer in length.